1. Field of the Invention
The present invention relates to electrocardiogram monitoring and, more particularly, to non-contacting electrodes that may work through a subject's clothing.
2. Description of the Related Art
Sensors and systems for electrocardiogram (ECG) measurements have been developed and refined for many years. The front-end of all ECG systems consists of the sensor electrodes and preamplifiers. Traditional adhesive sensor electrodes provide a stable, low impedance signal source that allow for the low noise measurement of ECG signals but the wires and adhesives employed are unsuitable for long term use; even common 24-48 hour Holter monitoring sessions are a considerable inconvenience and discomfort to the user so longer term personal health monitoring with traditional adhesive electrodes would be unworkable.
Non-contact biosensors for cardiac monitoring are of great interest for a number of long-term health sensing applications ranging from exercise and fitness monitoring to management of chronic health conditions, and may be built into emergency medical equipment, examining tables, beds or other furniture. However, the presence of motion-related artifacts and common mode interference remains a challenging problem for several reasons. First, interference from AC power mains may corrupt electrocardiograph signals due to low common mode rejection ratio (CMRR) of the sensor system. Interfering signals can be reduced by employing a driven right leg (DRL) connection, active shielding and guarding of cables, and by employing a notch filter at the output.
Second, triboelectrically generated static charge caused by rubbing between the electrodes and the subject's clothing is another problem as is common mode electrostatic charge on the subject. In addition to reducing movement at the electrode-subject interface, other ways to reduce this effect include the choice of materials that minimize static electric charge generation and providing a static charge discharge path at the electrode subject interface.
Third, the modulation of the bioelectric potential signals from motion-related source impedance changes presents a problem. The signal gain may be a function of the source capacitance and any stray capacitance at the preamplifier input, which can lead to baseline wandering and gain distortion. This effect can be minimized by employing a voltage mode preamplifier, as opposed to a charge-mode preamplifier, however the noise of the voltage mode preamplifier then becomes a consideration.
Finally, electronic noise interferes with the operation of non-contact biosensors. The preamplifier immediately following the ECG electrode is the major contributor to the overall electronic noise level, and may be addressed by employing careful preamplifier design.
Changes in source capacitance due to the relative motion of the electrodes and the subject leads to modulation of both the signals of interest as well as the above mentioned sources of interference and noise, which in turn may generate interference within the signal band of interest. This effect may be large enough in practical scenarios to completely obscure the ECG signal.
In addition, there has been some work on ECG data correction using adaptive filtering. In this work accelerometers were employed as ancillary sensors to monitor motion and then a digital correction was applied to the ECG signal by an adaptive filtering algorithm. However, the accelerometers were not attached at the precise point of the ECG electrodes and thus the accelerometer output was not strongly correlated to changes in the ECG electrode capacitance, thereby limiting the effectiveness of this method.
Traditionally, non-contact sensors are built with high impedance voltage follower amplifiers but, due to the small signal source capacitance, the input capacitance of the amplifier must be kept extremely small or otherwise neutralized, which degrades the noise performance. To cope with small bioelectric signals, a charge amplifier design may be used because the gain is independent of the input capacitance of the preamplifier. The disadvantage of a charge amplifier design is that its gain depends on the source capacitance, which may be modulated by the relative motion of the subject and the ECG electrodes. Other advantages of the charge amplifier configuration are that shielding is straightforward, and the low-frequency cutoff is independent of the source capacitance. In clinical ECG applications the driven right leg (DRL) technique is commonly used to achieve high common mode rejection ratio (CMRR); this technique can be employed with either voltage or current mode preamplifiers. However, the DRL method is not feasible in an ambulatory setting. Accordingly, there is a need for an alternative design of a non-contact ECG sensor that can lead to a more accurate correction method.